My previous patents, "Radio Receiver Operational Checking Method and System including Reactive Coupling", U.S. Pat. No. 5,241,698, and "Radio Receiver Noise Squelching and Operational Indicating Alarm", U.S. Pat. No. 4,996,717, addressed the problem of checking the operation of radio communications receivers. This invention relates to the checking of navigation equipment in a similar manner.
Flying an airplane without reference to outside landmarks, such as landing by the Instrument Landing System (ILS) or en-route navigation using the military TACAN or civilian VHF-Omirange (VOR) or Area Navigation (RNAV) system, is sometimes referred to as "Flying the needles" or "On the gauges". The pilot is in or above the clouds and cannot depend on his senses and vision to tell him what the airplane is doing. He must trust to gauges, usually electromechanical devices which are prone to failure.
The Instrument Landing System (ILS) directs the airplane along a threedimensional path in space which ends over the runway. The horizontal dimension is delineated by a radio signal called the Localizer signal. In the airplane the "localizer indicator" is a meter needle which moves to the left or right to tell the pilot which direction to correct for a perfect centered approach. Likewise a "glideslope needle" is a horizontal bar which moves up or down to tell the pilot to fly up or down. The needles are centered in mid position when the airplane is on the proper safe path to the runway.
The system wisely employs a indicator in the form of a red flag which drops into view when the all important ILS signals are not adequate. For a proper safe landing the airplane is flown so that the needles are centered and the flag is out of view, together showing that the airplane is on the correct vertical and horizontal paths.
Now here is the cause for concern. Stationary needles as just explained show that all is well with the approach or navigation. But stationary centered needles can also be the result of inactive malfunctioning indicators. This could be due to the needle movements themselves being stuck or disconnected, or the solid state component feeding control power to the needle indicators could have failed. A tripped circuit breaker or an opened connection will cause needles to center, and this has caused dangerous confusion. Unlikely as these things are, they are a dangerous possibility when relying on motionless needles or flags. While intently watching the needles a flyer might not notice if the no-signal flag has dropped, or the flag might well not drop if the signal is correct but the indicator or driver circuitry has failed.
Thus, the situation has been set up where perfect flying is indistinguishable from instrument failure.
This was exactly the case in a recent accident involving an Alitalia DC-9, in which the crew flew into the side of a mountain while following an inoperative glide slope indicator. (See "When the Glideslope Fails"; IFR Refresher; Belvoir Publications, Inc.; Vol. 8 Issue 3, March 1994).
Some pilots have attempted to compensate for this design flaw by always flying with the needles slightly off center. This will give some warning of failure, but it is not a satisfactory technique for instrument approaches or for rapidly-changing conditions. A means is needed to constantly monitor the condition of the instruments, while allowing the pilot to fly a perfect approach with the needles properly centered.
The object of this invention is to save lives by enhancing the safety of Aircraft navigation Instruments and specifically the Instrument Landing System indicators. This is accomplished by checking and monitoring the navigational indicating equipment. The danger addressed by this invention is that failure of the system due to any one critical part can occur unnoticed by the crew even though thoughtful design has gone into the system and the equipment.
This invention has approached the situation with these criteria:
1. To check the possible failure modes which exist in present equipment. PA1 2. To be inherently simple in concept. PA1 3. To do its job of checking with no added work load to the pilot(s). PA1 4. To be invisible to the existing aircraft circuitry and operation, causing no changes or shifts because of its employment. PA1 5. It should be simple to install. PA1 6. It should ideally be economical to manufacture and install.
These criteria have been addressed by this invention as will be shown.